1. Field of the Invention
The present invention relates to laser systems.
2. Description of the Related Art
In various industrial fields, semiconductor lasers are used as light sources emitting coherent light. For example, many semiconductor lasers are used in optical data storage devices. Under these industrial circumstances, demands for a light source capable of changing the wavelength of coherent light are gradually increasing. The fields in which demands for such a light source, for example, an optical data storage field includes holographic recording and reproducing apparatuses, serving as next-generation recording and reproducing apparatuses.
In the holographic recording and reproducing apparatuses, a signal beam and a reference beam are emitted from a single laser source. The signal beam is modulated on the basis of information (recording data) to be recorded for each page. The signal and reference beams are applied to a holographic recording medium. The signal beam interferes with the reference beam in the holographic recording medium, thus producing an interference fringe pattern in the medium. The interference fringe pattern is formed as a diffraction grating (hologram) based on a change in refractive index or transmittance of the holographic recording medium, thus recording the recording data. On the other hand, in a holographic reproducing apparatus for reproducing recording data from a diffraction grating (hologram) formed as described above, a photodetector detects a diffracted beam (reconstructed beam) generated by irradiation of the diffraction grating (hologram) formed in the recording medium with the reference beam, thus reproducing the recording data.
As a laser source used in such a holographic recording and reproducing apparatus, a nearly single-mode laser source is preferably used. According to a method, when the temperature of a holographic recording and reproducing apparatus varies, the wavelength of a light beam emitted from a laser source is changed to improve the recording and reproducing characteristics. This method is proposed by Mitsuru Toishi, et al. “Temperature Tolerance Improvement with Wavelength Tuning Laser Source in Holographic Data Storage”, Technical Digest of Optical Data Storage/International Symposium on Optical Memory 2005 ThE5. Light sources capable of changing the wavelength of coherent light are requested for the above-described application.
The above-described laser sources include a Littrow laser system. FIG. 10 shows an example of a Littrow laser system. In the Littrow laser system, a multi-mode laser diode (LD) 101 emits a light beam. A lens 103 collimates the light beam passing therethrough. A grating 102 is irradiated with the collimated light beam. The grating 102 is rotated about a support 105 through a screw 104 to adjust the angle of the grating 102 relative to the LD 101. A light beam having the same wavelength as that of a first-order beam, which is diffracted by the grating 102 and is returned to the LD 101, is selected and is oscillated, so that a zero-order beam is obtained. The above-described Littrow laser system is proposed by Tomiji Tanaka, et al. “Littrow-Type Blue Laser for Holographic Data Storage”, Technical Digest of Optical Data Storage 2004, p. 311. A Littman laser system is also known as another type of laser system. In the Littman laser system, the positional relationship between a diffraction grating and a laser diode is fixed and a light beam diffracted by the diffraction grating is returned to the laser diode by using a mirror.